Devices and charging systems of the above-named type are known from the related art. In the case of charging units which are connected to supply networks having limited resources, it is not always practical or possible to withdraw energy from the supply network. This is in particular the case when a further withdrawal of energy from the supply network would result in a complete exhaustion of the supply network. Such supply networks may be found, for example, in motor vehicles. Motor vehicles have a vehicle electrical system, which is used, for example, as a supply network for auxiliary units or other electrical equipment. The vehicle electrical system itself is also supplied with energy from a battery or a rechargeable energy store, so that the energy available to it is basically finite, unless the battery is recharged. If the load on the supply network is too high and the energy source is exhausted, further operation of the motor vehicle is no longer possible.
A charging unit connected to the supply network is then also no longer supplied with energy and a charging operation cannot take place. In this state, it is consequently also not possible to inform a user of this state. For the user, the state of the charging unit is thus not directly apparent. If it is apparent to the user that the charging operation is not taking place, the user might also assume from this that a defect is present, which may result in unnecessary irritation of the user.